It is known to measure the oxygen saturation of the haemoglobin in the arterial blood (arterial oxygen saturation) by means of a non-invasive method which is designated as pulse oximetry. This method is used for monitoring patients, e.g. during anaesthesia and intensive care. The principle of the measurement is based on the difference between the optical absorbtivities of haemoglobin in its form saturated with oxygen and its reduced form. In the case of red light, the absorption coefficient of blood is greatly dependent upon the oxygen content and, in the case of light in the near infrared range, almost independent thereof. By measuring the ratio of the intensities of the absorbed light of both wavelengths, it is possible to determine the arterial oxygen saturation.
In pulse oximetry, as a rule the light sources employed are two closely mutually adjacent light emitting diodes (LED) having wavelengths of approximately 660 nm (red) and approximately 890 nm (infrared). The light emitted by the two LEDs is passed into a body part (e.g. the pad of the finger) which is well supplied with blood, and is there scattered and partially absorbed. The light emerging is measured by a photodiode which, as a rule, is disposed to be situated opposite the LEDs. The LEDs and the photodiode are usually integrated in an assembly which is designated as a pulse oximetric sensor. The separate measurement of the red and infrared light using only one photodiode is made possible by the use of alternating light pulses of the two wavelengths, which are separately metrologically picked up and evaluated.
The light of both wavelengths which is measured by the photodiode consists of a steady and a time dependent component. The steady component is essentially determined by absorption by bones, tissue, skin and non-pulsating blood. The time dependent component is caused by changes in absorption in the specimen under test, which, in the ideal case, are caused only by the arterial blood flowing in in pulsed fashion. To determine the arterial oxygen saturation (SaO.sub.2), the steady components (DC.sub.R, DC.sub.IR) and the time dependent components (AC.sub.R, AC.sub.IR) of the measured red (R) and infrared (IR) light intensities are utilized. Usually, the arterial oxygen saturation is determined using the relation: ##EQU1##
where f represents an empirically determined function.
A problem which has not yet been satisfactorily solved in pulse oximetric measurement resides in that disturbances to the measurement signals which are caused by movements of the patient or his environment cannot be eliminated entirely. Such disturbances are critical particularly in circumstances in which they occur periodically, since in this case, they may lead under specified conditions to false measurement results. Since the frequency distribution of movement artifacts may overlap that of the physiological signal, conventional band pass filters or selective filters are not suitable for reliably separating movement artifacts from the physiological signal. Even adaptive filter techniques, such as for example the method of adaptive spurious frequency suppression, cannot be directly applied to pulse oximetry, since these presuppose that either the spurious frequencies or the physiological signals exhibit predictable frequency characteristics. This prerequisite is not satisfied either in respect of movement artifacts or in respect of the pulse frequency. In particular in the case of patients having cardiovascular disorders, the latter may exhibit a high variability.
In principle, it has to be stated that, as a consequence of the nature of the problem, limits are set to any solution which is based solely on an improvement of the signal processing. These limits are caused by the fact that disturbances due to movement artifacts cannot be entirely eliminated, since they are not always detected as such. Primarily, it is accordingly necessary to seek a solution via the route of a differentiated signal extraction which permits a separation of the spurious signals from the physiological signals. Various solution routes have already been proposed in this sense.
WO-A-94/03102 contains a description of an optical monitoring device which comprises:
a) a sensor having a transmitter part which has three light emitting diodes which emit light of differing wavelength, and having a receiver part to measure the light intensity, which receiver part has three photodetectors, and PA1 b) a control and evaluating part.
To suppress the spurious signals caused by movements of a patient or his environment, it is proposed to normalize the signals generated at the photodetectors to equal levels of their DC components. Proceeding on the basis of the assumption that the amplitudes of the spurious components of these normalized signals are equally large, the differences of the normalized signals are then formed to eliminate the spurious components. However, in practice the amplitudes of the spurious components of the normalized signals are different, as is also mentioned on page 6, lines 4 to 6 of this publication. Accordingly, a complete suppression of spurious signals does not take place.
U.S. Pat. No. 4,802,486 contains a description, for example, of a method which is based on using specified measurement signals, derived from the ECG of the patient, to identify the arterial pulsations. Signals which are not identified as such (i.e. spurious signals) can thus be suppressed. This method, which is designated as ECG-synchronized pulse oximetry, has the disadvantage that spurious signals which occur simultaneously with a pulse signal are not picked up. Moreover, the simultaneous measurement of the ECG is presumed. However, this is not always available.
In U.S. Pat. No. 5,226,417, it is proposed to incorporate into the pulse oximetric sensor a measured value pickup which detects movements at the location of the pulse oximetric test point. Piezoelectric films, acceleration transducers and wire strain gauges are mentioned as examples of such measured value pickups. However, such a solution demands a considerable expenditure in the manufacture of the sensor; this leads to a substantial increase in the cost of the product, which is often designed as a disposable article. Moreover, by reason of the extremely stringent requirements imposed on the sensitivity of the movement detection, a considerable expenditure in respect of signal processing is necessary.
A similar idea is described in U.S. Pat. No. 5,025,791. In that document, it is proposed to incorporate into the pulse oximetric sensor a movement detector which is specifically designed for this purpose and which is based on an electromechanical or a magnetic measurement method or a combination of both of these methods. The objections which have already been set forth hereinabove are factors against such a concept.
Another solution is proposed in WO-A-91/18550. That document contains a description of an arrangement which is provided for the measurement of the pulse frequency, the concept of which could however also be transferred to pulse oximetry. In a sensor, which is applied to the forehead of a person, there are incorporated a LED emitting in the infrared and a LED emitting in the yellow frequency range as well as two photodiodes. The light emitted into the tissue of the forehead is back-scattered there and measured by the two photodiodes. The signal generated by the infrared light contains components which are caused both by the pulsating arterial blood and also by movements. In contrast, the signal generated by the yellow light is to a large extent independent of blood pulsations and contains only the components caused by movements. This may be explained in that infrared light is able to penetrate deeply into the forehead tissue which is well supplied with blood, while yellow light has a substantially smaller depth of penetration and therefore picks up only processes in the vicinity of the surface of the skin of the forehead, i.e. in a region whose blood supply is weak. The two signals can now be analyzed by means of known processes, and the components of the infrared signal which are caused by movements can be removed. In the case of a transfer of this concept to pulse oximetry, it has to be borne in mind that, for reasons which are predominantly practical, but also physiological, the forehead lacks suitability as a test point. The test point which is most frequently used for pulse oximetric measurements is the finger. Measurements are often made also on the ear lobe and on the toe. It is common to these three test points that even the tissue parts which are close to the surface have a good supply of blood. There, a separation of the signal components caused by pulsations and by movements is accordingly not readily possible by the use of light of differing depths of penetration.